Reductant injecting device, exhaust gas processing device and exhaust gas processing method

ABSTRACT

A reductant injecting device, including: a honeycomb structure including: a pillar shaped honeycomb structure portion having partition wall that defines a plurality of cells each extending from a fluid inflow end face to a fluid outflow end face; and at least one pair of electrode portions arranged on a side surface of the honeycomb structure portion; an outer cylinder having an inlet side end portion and an outlet side end portion, the inlet side end portion comprising a carrier gas introduction port being configured to introduce a carrier gas, the outlet side end portion comprising an injection port being configured to inject ammonia; a urea sprayer arranged at one end of the outer cylinder; and a spray direction switcher configured to be able to switch a spray direction of the aqueous urea solution.

FIELD OF THE INVENTION

The present invention relates to a reductant injecting device, an exhaust gas processing device, and an exhaust gas processing method.

BACKGROUND OF THE INVENTION

An exhaust gas processing device using a selective catalytic reduction NOx catalyst (SCR catalyst) is known for purifying nitrogen oxides (NOx) in exhaust gases discharged from various engines (Patent Literature 1).

The exhaust gas processing device described in Patent Literature 1 includes a catalyst (SCR catalyst) attached to an exhaust gas pipe of an engine and a means for injecting urea water into the exhaust gas pipe between the engine and the catalyst, and also includes a plurality of urea water injection means for mixing urea water with an exhaust gas, reacting with specific components in the exhaust gas by the catalyst, and mixing the urea water with the exhaust gas.

However, in the exhaust gas processing device described in Patent Literature 1, a temperature of the exhaust gas has to be 200° C. or more in order to decompose urea in the urea water into ammonia by the heat of the exhaust gas. Therefore, when the temperature of the exhaust gas is lower, there is a problem that the decomposition reaction of urea is difficult to take place, and an amount of ammonia required for the NOx treatment is insufficient.

Therefore, an exhaust gas processing device using a reductant injecting device has been proposed, wherein the reductant injecting device includes: a honeycomb structure (a honeycomb heater) having a cylindrical honeycomb structure portion and a pair of electrode portions arranged on a side surface of the honeycomb structure portion; and a urea sprayer being configured to spray an aqueous urea solution onto the honeycomb structure portion (Patent Literature 2). The reductant injecting device used in the exhaust gas processing device can spray the aqueous urea solution onto the honeycomb structure portion that has been electrically heated by applying a voltage to the electrode portions, and decompose the urea in the aqueous urea solution in the honeycomb structure to produce ammonia efficiently.

However, the spraying of the aqueous urea solution onto the honeycomb structure portion electrically heated decreases a temperature of a region where the aqueous urea solution is sprayed, thereby generating a temperature irregularity in the honeycomb structure portion. As a result, urea deposits (crystals caused by the urea) tend to be generated in a lower temperature region of the honeycomb structure portion. The generation of the urea deposits blocks a flow path in the honeycomb structure portion, which will inhibit the decomposition of the urea into ammonia.

Therefore, exhaust gas processing devices using a reductant injecting device provided with a carrier gas introduction port between the urea injector and the honeycomb structure have been proposed (Patent Literatures 3 and 4). According to the reductant injecting device used in each exhaust gas processing device, a carrier gas introduced from the carrier gas introduction port can facilitate the flow of the gas in the honeycomb structure portion. Therefore, even if the aqueous urea solution is sprayed onto the honeycomb structure portion, a temperature difference in the honeycomb structure portion can be decreased, thereby suppressing the generation of urea deposits.

CITATION LIST Patent Literatures

-   [Patent Literature 1] Japanese Patent Application Publication No.     2007-327377 A -   [Patent Literature 2] WO 2014/148506 -   [Patent Literature 3] Japanese Patent Application Publication No.     2017-180298 A -   [Patent Literature 4] Japanese Patent Application Publication No.     2017-180299 A

SUMMARY OF THE INVENTION

The present invention relates to a reductant injecting device, comprising:

a honeycomb structure comprising:

-   -   a pillar shaped honeycomb structure portion having partition         wall that defines a plurality of cells each extending from a         fluid inflow end face to a fluid outflow end face; and     -   at least one pair of electrode portions being configured to heat         the honeycomb structure portion by conducting a current, the         pair of the electrode portions being arranged on a side surface         of the honeycomb structure portion, the honeycomb structure         being configured to be able to decompose urea in an aqueous urea         solution in the honeycomb structure portion heated by conducting         the current to generate ammonia;

an outer cylinder having an inlet side end portion and an outlet side end portion, the inlet side end portion comprising a carrier gas introduction port being configured to introduce a carrier gas, the outlet side end portion comprising an injection port being configured to inject ammonia, wherein the outer cylinder houses the honeycomb structure, and a first flow path through which a fluid can flow through the plurality of cells and a second flow path through which the fluid can flow from the inlet side end portion to the outlet side end portion without passing through the plurality of cells are provided in the outer cylinder;

a urea sprayer being configured to spray the aqueous urea solution into the first flow path or the second flow path on an inflow side of the fluid, the urea sprayer being arranged at one end of the outer cylinder; and

a spray direction switcher configured to be able to switch a spray direction of the aqueous urea solution to the first flow path or the second flow path depending on temperatures of an exhaust gas.

Further, the present invention relates to an exhaust gas processing device, comprising:

an exhaust gas pipe through which an exhaust gas flows;

the reductant injecting device being configured to inject ammonia or an aqueous urea solution into the exhaust gas pipe; and

an SCR catalyst arranged at the exhaust cylinder on a downstream side of a position where ammonia or the aqueous urea solution is injected.

Furthermore, the present invention relates to a method for processing an exhaust gas, the method comprising:

injecting an aqueous urea solution into the exhaust gas from the reductant injecting device, and reducing the exhaust gas mixed with the aqueous urea solution by an SCR catalyst, when a temperature of the exhaust gas is 200° C. or higher; and

injecting generated ammonia into the exhaust gas by the reductant injecting device, and reducing the exhaust gas mixed with the ammonia by the SCR catalyst, when a temperature of the exhaust gas is lower than 200° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing a reductant injecting device according to Embodiment 1 of the present invention;

FIG. 2 is a schematic cross-sectional view taken along the line A-A′ in FIG. 1 ;

FIG. 3 is a schematic cross-sectional view showing a reductant injecting device according to Embodiment 2 of the present invention;

FIG. 4 is a schematic cross-sectional view taken along the line B-B′ in FIG. 3 ;

FIG. 5 is a schematic cross-sectional view taken along the line C-C′ in FIG. 3 ;

FIG. 6 is a schematic cross-sectional view showing an exhaust gas processing device according to Embodiment 3 of the present invention; and

FIG. 7 is a schematic cross-sectional view showing another exhaust gas processing device according to Embodiment 3 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An amount of NOx in an exhaust gas is changed depending on driving patterns of motor vehicles. When the amount of NOx in the exhaust gas is higher, an amount of a reductant (ammonia) required for purifying NOx is increased, so that an amount of the aqueous urea solution sprayed is needed to be increased. The increased amount of the aqueous urea solution sprayed decreases a temperature of the honeycomb structure, so that an electric power for conducting a current is required to be increased, resulting in an increased power consumption.

Further, in general, the amount of the reductant required for purifying NOx depends on temperatures of the exhaust gas containing NOx. When the temperature of the exhaust gas is lower, the amount of the reductant required is decreased, and when the temperature of the exhaust gas is higher, the amount of reductant required is increased. Further, when the temperature of the exhaust gas is higher (for example, 200° C. or higher), the urea is decomposed by the heat of the exhaust gas to generate ammonia even if the aqueous urea solution is directly sprayed into the exhaust gas, so that NOx can be purified. Therefore, when the temperature of the exhaust gas is lower, ammonia can be sprayed from the reductant injecting device, and when the temperature of the exhaust gas is higher, the aqueous urea solution can be sprayed from the reductant injecting device.

However, the conventional reductant injecting device described in each of Patent literatures 2 to 4 is not intended to spray the aqueous urea solution as it is. Therefore, when the amount of the aqueous urea solution sprayed is increased, the honeycomb structure portion will hinder the flow of the aqueous urea solution. This results in problems that the aqueous urea solution cannot be efficiently injected from the reductant injecting device, and the urea deposits are easily generated.

The present invention has been made to solve the above problems. An object of the present invention is to provide a reductant injecting device which has decreased power consumption and can select ammonia or an aqueous urea solution depending on temperatures of the exhaust gas to inject the ammonia or the aqueous urea solution efficiently.

Also, a further object of the present invention is to provide an exhaust gas processing device and an exhaust gas processing method, which can efficiently injecting ammonia or an aqueous urea solution from the reductant injecting device depending on temperatures of the exhaust gas to purify NOx.

As a result of intensive studies for a structure of a reductant injecting device including a honeycomb structure in which at least one pair of electrode portions are arranged on a side surface of a honeycomb structure portion, the present inventors have found that the above problems can be solved by forming a first flow path through which a fluid can flow in cells of the honeycomb structure portion and a second flow path through which the fluid can flow without passing through the cells of the honeycomb structure portion in an outer cylinder for housing the honeycomb structure, and providing a spray direction switcher that can spray an aqueous urea solution in the first flow path or the second flow path depending on temperatures of the exhaust gas, and they have completed the present invention.

According to the present invention, it is possible to provide a reductant injecting device which has decreased power consumption and can select ammonia or an aqueous urea solution depending on temperatures of an exhaust gas to inject the ammonia or the aqueous urea solution efficiently.

Also, according to the present invention, it is possible to provide an exhaust gas processing device and an exhaust gas processing method, which can efficiently injecting ammonia or an aqueous urea solution from the reductant injecting device depending on temperatures of an exhaust gas to purify NOx.

Hereinafter, preferable embodiments of a reductant injecting device, an exhaust gas processing device and a method for processing an exhaust gas according to the present invention will be specifically described with reference to the drawings. It is to understand that the present invention is not limited to the following embodiments, and those which appropriately added changes, improvements and the like to the following embodiments based on knowledge of a person skilled in the art without departing from the spirit of the present invention fall within the scope of the present invention. For example, some components may be deleted from all components set forth in the embodiments, or components of different embodiments may be combined as appropriate.

Embodiment 1

(1) Reductant Injecting Device

FIG. 1 is a schematic cross-sectional view showing a reductant injecting device according to Embodiment 1 of the present invention (a schematic cross-sectional view parallel to an extending direction of cells of a honeycomb structure portion). Further, FIG. 2 is a schematic cross-sectional view taken along the line A-A′ in FIG. 1 (a schematic cross-sectional view perpendicular to the extending direction of the cells of the honeycomb structure portion).

As shown in FIGS. 1 and 2 , a reductant injecting device 100 according to the present embodiment includes: a honeycomb structure 1; an outer cylinder 2 housing the honeycomb structure 1; and a urea sprayer 3 arranged at one end of the outer cylinder 2.

The honeycomb structure 1 includes: a pillar shaped honeycomb structure portion 11 having partition wall 15 that defines a plurality of cells 14 extending from a fluid inflow end face 13 a to a fluid outflow end face 13 b; and at least one pair of electrode portions 12 being configured to heat the honeycomb structure portion 11 by conducting a current, the electrode portions 12 being arranged on a side surface of the honeycomb structure portion 11. The honeycomb structure portion 11 has a hollow structure configured such that a space region is located at a center in a cross section perpendicular to the extending direction of the cells 14. In the hollow honeycomb structure portion 11 having such a structure, the cells 14 form a first flow path and the space region forms a second flow path 16.

As used herein, the “fluid inflow end face 13 a” means an end face having a fluid inflow port, and the “fluid outflow end face 13 b” means an end face having a fluid outflow port. Further, from the viewpoint of uniformly heating the honeycomb structure portion 11, the “pair of electrode portions 12” are preferably arranged such that one electrode portion 12 is arranged on an opposite side of the other electrode portion 12 across the center of the honeycomb structure portion 11, in a cross section orthogonal to the extending direction of the cells 14 of the honeycomb structure portion 11.

The electrode portion 12 is preferably formed in a band shape along the extending direction of the cells 14. One pair of electrode portions 12 are preferred, but pairs of electrode portions 12 are possible in terms of increasing a heat generation efficiency of the honeycomb structure portion 11.

The outer cylinder 2 has an inlet side end portion and an outlet side end portion, and houses the honeycomb structure 1 therein. The inlet side end portion, which is one end, is provided with a carrier gas introduction port 22 being configured to introduce a carrier gas. The inlet side end portion is also provided with the urea sprayer 3. The outlet side end, which is the other end, is provided with an injection port 21 being configured to inject ammonia.

In the interior of the outer cylinder 2, a first flow path through which a fluid can flow in the cells 14 of the honeycomb structure portion 11 and a second flow path through which the fluid can flow from the inlet side end portion to the outlet side end portion without passing through the cells 14 are provided. That is, in the interior of the outer cylinder 2, the first flow path (cells 14) and the second flow path 16 are arranged in parallel in the length direction of the outer cylinder 2. Since the second flow path 16 is configured such that the fluid can flow through a portion other than the cells 14 of the honeycomb structure portion 11, the fluid flows through either the first flow path (cells 14) or the second flow path 16.

The carrier gas introduction port 22 is preferably provided on the fluid inflow end surface 13 a side of the honeycomb structure portion 11, that is, between the honeycomb structure 1 and the urea sprayer 3. Non-limiting examples of the carrier gas that can be used include exhaust gases, intake gases, and air from other air feeding devices (compressors and the like mounted on large vehicles and the like).

The honeycomb structure 1 housed in the outer cylinder 2 is fixed (held) in the outer cylinder 2 via an insulation holding portion 23. This can allow insulation between the honeycomb structure 1 and the outer cylinder 2 to be ensured. There may be a portion (space) where the insulation holding portion 23 is not arranged between the honeycomb structure 1 and the outer cylinder 2, but the entire outer circumference of the honeycomb structure 1 may be covered with the insulation holding portion 23. A material of the insulation holding portion 23 is not particularly limited as long as it has excellent insulating properties, and for example, alumina, glass, or the like can be used.

The urea sprayer 3 is arranged at one end (inlet side end portion) of the outer cylinder 2 and sprays an aqueous urea solution on the fluid inflow end surface 13 a side of the honeycomb structure portion 11. Further, the urea sprayer 3 has a spray direction switcher configured to be able to switch a spray direction of the aqueous urea solution to the first flow path (cells 14) or the second flow path 16 depending on temperatures of the exhaust gas.

The reductant injecting device 100 according to the present embodiment, which has the above structure, sprays the aqueous urea solution from the urea sprayer 3 into the first flow path (cells 14) or the second flow path 16 depending on the temperatures of the exhaust gas. Urea in the aqueous urea solution sprayed into the first flow path (cells 14) is decomposed by the honeycomb structure portion 11 heated by conducting the current in the first flow path (cells 14) to generate ammonia (a reductant), and the ammonia is injected to the outside via the injection port 21. On the other hand, the aqueous urea solution sprayed into the second flow path 16 is directly injected to the outside via the injection port 21. In this case, the introduction of the carrier gas into the outer cylinder 2 can create the flow of the gas in the first flow path (cells 14) and the second flow path 16. This prevents the aqueous urea solution from being stagnated in the first flow path (cells 14) and the second flow path 16, thereby suppressing urea deposits.

Hereinafter, the reductant injecting device 100 according to the present embodiment will be described in detail for each component.

(1-1) Honeycomb Structure 1

The honeycomb structure 1 includes the honeycomb structure portion 11 and the electrode portions 12.

The partition wall 15 forming the honeycomb structure portion 11 may preferably be made of ceramics, although not particularly limited thereto. In particular, the partition wall 15 preferably contains a silicon-silicon carbide composite material or silicon carbide as a main component, and more preferably a silicon-silicon carbide composite material as a main component. The use of such a material can allow electrical resistivity of the honeycomb structure portion 11 to be easily adjusted to any value by changing a ratio of silicon carbide and silicon.

As used herein, the “silicon-silicon carbide composite material” as the main component means a material containing silicon carbide particles as an aggregate and metallic silicon as a binding material for binding silicon carbide particles. In the silicon-silicon carbide composite material, it is preferable that a plurality of silicon carbide particles are bonded by metallic silicon. Further, the “silicon carbide” as the main component means a material formed by sintering silicon carbide particles. Furthermore, as used herein, the “main component” means a component contained in an amount of 90% by mass or more.

The honeycomb structure portion 11 preferably has an electrical resistivity of from 0.01 to 500 Ωcm, and more preferably from 0.1 to 200 Ωcm, although not particularly limited thereto. The control of the electrical resistivity to such a level can effectively heating the honeycomb structure portion 11 by applying a voltage to at least one pair of electrode portions 12. More particularly, in order to heat the honeycomb structure portion 11 to 160 to 600° C. using a power source having a voltage of from 12 to 200 V, the electrical resistivity is preferably in the above range.

The electrical resistivity of the honeycomb structure portion 11 is a value at 25° C. The electrical resistivity of the honeycomb structure portion 11 is a value measured by a four-terminal method.

The honeycomb structure portion 11 preferably has a surface area per unit volume of from 5 cm²/cm³ or more, and more preferably from 8 to 45 cm²/cm³, and particularly preferably from 20 to 40 cm²/cm³. The surface area of 5 cm²/cm³ or more can allow a sufficient contact area with the aqueous urea solution to be ensure, thereby appropriately controlling a treatment rate of the aqueous urea solution, i.e., an amount of ammonia generated (a generation rate).

The surface area of the honeycomb structure portion 11 is an area of the surfaces of the partition wall 15 of the honeycomb structure portion 11.

The partition wall 15 of the honeycomb structure portion 11 preferably have a thickness of from 0.06 to 1.5 mm, and more preferably from 0.10 to 0.80 mm. The thickness of the partition wall 15 of 1.5 mm or less can reduce a pressure loss, thereby appropriately controlling the treatment rate of the aqueous urea solution, i.e., the amount of ammonia generated (generation rate). The thickness of the partition wall 15 of 0.06 mm or more can prevent the honeycomb structure portion 11 from being destroyed by a thermal shock caused by electric conduction heating.

When the shape of each cell 14 (the shape of the cross section orthogonal to the extending direction of the cell 14) is circular as shown in FIG. 2 , the thickness of the partition wall 15 means a thickness of a portion where “a distance between the cells 14 is the shortest (a portion where the thickness of the partition wall 15 is lower)”.

The cells 14 preferably have a density of from 7 to 140 cells/cm², and more preferably from 15 to 120 cells/cm². The density of the cells 14 of 7 cells/cm² or more can allow a sufficient contact area with the aqueous urea solution to be ensured, thereby appropriately controlling the treatment rate of the urea aqueous solution, i.e., the amount of ammonia generated (generation rate). The density of the cells 14 of 140 cells/cm² or less can reduce the pressure loss, thereby appropriately controlling the treatment rate of the aqueous urea solution, i.e., the amount of ammonia generated (generation rate).

The honeycomb structure portion 11 may have some cells 14 provided with plugged portions at the end portion on the fluid inflow end surface 13 a side. The material of the plugged portions is preferably the same as that of the partition wall 15, but other materials may be used.

A shape (outer shape) of the fluid inflow end face 13 a is not particularly limited, and it may be various shapes such as a square, a rectangle, or other polygons, and an ellipse in addition to the circular shape as shown in FIG. 2 . Further, the shape (outer shape) of the fluid inflow end surface 13 a is the same as that of the fluid outflow end face 13 b, and preferably as the shape (outer shape) of the cross section orthogonal to the extending direction of the cells 14.

The size of the honeycomb structure portion 11 is such that the areas of the fluid inflow end surface 13 a and the fluid outflow end surface 13 b including the second flow path 16 (the area of the cross section orthogonal to the extending direction of the cells 14) are from 50 to 10000 mm², respectively, and more preferably from 100 to 8000 mm², respectively. Further, the area of the second flow path 16 in the cross section orthogonal to the extending direction of the cells 14 is preferably from 20 to 2000 mm².

The shape of each cell 14 in the cross section orthogonal to the extending direction of the cell 14 is preferably an ellipse, a quadrangle, a hexagon, an octagon, or a combination thereof, in addition to the circular shape as shown in FIG. 2 . Such a shape can reduce the pressure loss when the exhaust gas is passed through the honeycomb structure portion 11, thereby efficiently decomposing the urea in the aqueous urea solution.

The shape of the second flow path 16 provided at the center of the honeycomb structure portion 11 may be, for example, in addition to the circle as shown in FIG. 2 , various shapes such as a square, a rectangle, or other polygons, and ellipses, in the cross section perpendicular to the extending direction of the cells 14, although not particularly limited thereto.

The shape (outer shape) of the honeycomb structure portion 11 and the shape of the second flow path 16 may be the same or different, but they are preferably the same in terms of resistance to external impact, thermal stress, and the like.

Each electrode portion 12 is formed in a band shape along the extending direction of the cells 14, but it may be formed in a wider width extending in the circumferential direction of the honeycomb structure portion 11. Further, in the cross section orthogonal to the extending direction of the cells 14, one electrode portion 12 is arranged on the opposite side of the other electrode portion 12 with the center of the honeycomb structure portion 11 interposed therebetween. Such a configuration can allow any bias of the current flowing in the honeycomb structure portion 11 to be suppressed when the voltage is applied between the pair of electrode portions 12, so that the bias of heat generation in the honeycomb structure portion 11 can be suppressed.

Further, the application of the voltage to the electrode portions 12 preferably heats the honeycomb structure portion 11 such that the temperature of the fluid inflow end surface 13 a is 900° C. or less. The temperature of the honeycomb structure portion 11 can be controlled by directly providing a temperature measuring means on the honeycomb structure portion 11. Alternatively, it is also possible to estimate and control the temperature of the honeycomb structure portion 11 from a temperature of the carrier gas, a flow rate of the carrier gas, and the amount of the aqueous urea solution sprayed. Further, if operating conditions of the engine are mapped, the mapping may be replaced with the measurement of the temperature of the carrier gas and the flow rate of the carrier gas.

The material of the electrode portions 12 is preferably the same as the main component of the partition wall 15 of the honeycomb structure portion 11, although not particularly limited thereto.

The electrode portions 12 preferably have an electrical resistivity of from 0.0001 to 100 Ωcm, and more preferably from 0.001 to 50 Ωcm. The electrical resistivity of the electrode portions 12 in such a range can allow the pair of electrode portions 12 to effectively play the role of electrodes in an exhaust gas pipe through which an exhaust gas having elevated temperature flows. The electrical resistivity of the electrode portions 12 is preferably lower than that of the honeycomb structure portion 11.

The electrical resistivity of the electrode portions 12 is a value at 400° C. The electrical resistivity of the electrode portions 12 is a value measured by the four-terminal method.

The pair of electrode portions 12 may be provided with electrode terminal protruding portions 17 for connecting electrical wirings 19 from the outside. The material of the electrode terminal protruding portions 17 may be conductive ceramics or a metal. Further, the material of the electrode terminal protruding portions 17 is preferably the same as that of the electrode portions 12. Further, it is preferable that each electrode terminal protruding portion 17 and a connector 18 of the outer cylinder 2 are connected by the electric wiring 19.

The honeycomb structure portion 11 may be provided with a urea hydrolysis catalyst. By using the urea hydrolysis catalyst, ammonia can be efficiently produced from urea. Examples of the urea hydrolysis catalyst include titanium oxide and the like.

(1-2) Outer Cylinder 2

The outer cylinder 2 is preferably made of stainless steel or the like, although not particularly limited thereto.

In order fit the outer cylinder 2 to the honeycomb structure 1, the outer cylinder 2 preferably has the same type of shape as that of the honeycomb structure portion 11 in the cross section orthogonal to the extending direction of the cells 14. As use herein, “the same type of shape” means that when the shape of the outer cylinder 2 is square, the shape of the honeycomb structure portion 11 is also square, and when the shape of the outer cylinder 2 is rectangular, the shape of the honeycomb structure portion 11 is also rectangular. For example, when the shapes of the outer cylinder 2 and the honeycomb structure portion 11 are of the same type and their shapes are rectangular, it is not necessary for both to have the same ratio of the length to the width.

(1-3) Urea Sprayer 3

The urea sprayer 3 has a spray direction switcher configured to be able to switch the spray direction of the aqueous urea solution to the first flow path (cells 14) or the second flow path 16 depending on the temperatures of the exhaust gas.

Non-limiting examples of the spray direction switcher that can be used include a nozzle capable of changing the spray direction of the aqueous urea solution depending on feed pressures of the aqueous urea solution. Specific examples of the spray direction switcher that can be used include a nozzle capable of spraying the aqueous urea solution into the second flow path 16 when the feed pressure of the aqueous urea solution is lower, and capable of spraying the aqueous urea solution into the first flow path (cells 14) when the feed pressure of the aqueous urea solution is higher. In addition, when the spray direction switcher is used, a small amount of a part of the aqueous urea solution injected from the urea sprayer 3 may flow in the first flow path when the aqueous urea solution is sprayed into the second flow path 16 by the spray direction switcher.

The type of the urea sprayer 3 is not particularly limited as long as it can spray the aqueous urea solution. It is preferably a solenoid type, an ultrasonic type, a piezoelectric actuator type, or an atomizer type. By using these, the aqueous urea solution can be easily sprayed in the form of mists. Further, among these, the use of the solenoid type, the ultrasonic type, or the piezoelectric actuator type can allow the aqueous urea solution to be sprayed in the form of mists without using air. This can eliminate necessity of heating the air used for spraying the aqueous urea solution, whereby an amount of energy to be heated can be reduced. Further, since the injection volume is reduced by not using the air used for spraying, a speed at which the aqueous urea solution in the form of mists passes through the honeycomb structure portion 11 can be reduced, resulting in a prolonged reaction time required for decomposition. The size (diameter) of each droplet of the aqueous urea solution sprayed from the urea sprayer 3 is preferably 0.3 mm or less. If the size of the droplet is larger than 0.3 mm, it may be difficult to vaporize it when heated in the honeycomb structure portion 11.

Here, the solenoid type urea sprayer 3 is a device that sprays the aqueous urea solution by vibrating the solenoid or moving a piston back and forth by an electric field using the solenoid. Further, the ultrasonic type urea sprayer 3 is a device that sprays the aqueous urea solution in the form of mists by ultrasonic vibration. Furthermore, the piezoelectric actuator type urea sprayer 3 is a device that sprays the aqueous urea solution in the form of mists by vibration of a piezoelectric element. Moreover, the atomizer type urea sprayer 3 is, for example, a device that sprays the solution by sucking the solution with a pipe and blowing off the solution sucked up to an opening at the tip of the pipe in the form of mists using air. The atomizer type urea sprayer 3 may be a device in which a plurality of small openings are formed at the tip of the nozzle and the solution is sprayed in the form of mists from the openings.

For the urea sprayer 3, the spray direction (the direction where the droplets eject) of the aqueous urea solution is preferably directed to the fluid inflow end surface 13 a side of the honeycomb structure portion 11, in order to facilitate the spraying of the aqueous urea solution on the fluid inflow end surface 13 a side of the honeycomb structure portion 11.

Next, the method for producing the reductant injecting device 100 according to the present embodiment will be described in detail.

(2) Method for Producing Reductant Injecting Device 100

(2-1) Production of Honeycomb Structure 1

When the honeycomb structure 1 is made of ceramics, the method for producing the honeycomb structure 1 is preferably as follows:

The method for producing the honeycomb structure 1 includes: a production step of a honeycomb formed body; a production step of a honeycomb dried body; a production step of a honeycomb body with unfired electrodes, and a production step of a honeycomb structure.

(Production Step of Honeycomb Formed Body)

The step of a honeycomb formed body preferably include extruding a forming raw material to produce a honeycomb formed body. The forming raw material preferably contains a ceramic raw material and an organic binder. In addition to the ceramic raw material and the organic binder, the forming raw material may further contain a surfactant, a sintering aid, a pore former, water, and the like. The forming raw material can be obtained by mixing these raw materials.

The ceramic raw material in the forming raw material is “ceramics” or “a raw material that will form ceramics by firing”. In any case, the ceramic raw material will form ceramics after firing. The ceramic raw material in the forming raw material preferably contains metallic silicon and silicon carbide particles (silicon carbide powder) as main components, or silicon carbide particles (silicon carbide powder) as a main component. This can provide the resulting honeycomb structure 1 with conductivity. The metallic silicon is also preferably metallic silicon particles (metallic silicon powder). The phrase “contains metallic silicon and silicon carbide particles as main components” means that the total mass of the metallic silicon and silicon carbide particles is 90% by mass or more of the whole (ceramic raw material). Examples of components other than the main components contained in the ceramic raw material include SiO₂, SrCO₃, Al₂O₃, MgCO₃, and cordierite.

When the silicon carbide is used as the main component of the ceramic raw material, the silicon carbide is sintered by firing. Further, when the metallic silicon and the silicon carbide particles are used as the main components of the ceramic raw material, the silicon carbide particles as an aggregate are bonded to each other with the metallic silicon as a binder by firing.

When the silicon carbide particles (silicon carbide powder) and the metal silicon particles (metal silicon powder) are used as the ceramic raw materials, the mass of the metal silicon particles is preferably from 10 to 40% by mass, based on the total mass of the silicon carbide particles and the metal silicon particles. The silicon carbide particles preferably have an average particle size of from 10 to 50 μm, and more preferably from 15 to 35 μm. The metallic silicon particles preferably have an average particle size of from 0.1 to 20 μm, and more preferably from 1 to 10 μm. The average particle size of each of the silicon carbide particles and the metal silicon particles is a value measured by a laser diffraction method.

Examples of the organic binder include methyl cellulose, glycerin, and hydroxypropyl methyl cellulose. As the organic binder, one type of organic binder may be used, or a plurality of types of organic binders may be used. An amount of the organic binder blended is preferably from 5 to 10 parts by mass, when the total mass of the ceramic raw materials is 100 parts by mass.

As the surfactant, ethylene glycol, dextrin and the like can be used. As the surfactant, one type of surfactant may be used, or a plurality of types of surfactants may be used. An amount of the surfactant blended is preferably from 0.1 to 2.0 parts by mass, when the total mass of the ceramic raw materials is 100 parts by mass.

The sintering aid that can be used includes SiO₂, SrCO₃, Al₂O₃, MgCO₃, cordierite and the like. As the sintering aid, one type of sintering aid may be used, or a plurality of types of sintering aids may be used. An amount of the sintering aid blended is preferably from 0.1 to 3 parts by mass, when the total mass of the ceramic raw materials is 100 parts by mass.

The pore former is not particularly limited as long as it forms pores after firing. Examples include graphite, starch, foamed resins, water-absorbent resins, and silica gel. As the pore former, one type of pore former may be used, or a plurality of types of pore formers may be used. An amount of the pore former blended is preferably from 0.5 to 10 parts by mass, when the total mass of the ceramic raw materials is 100 parts by mass.

An amount of water blended is preferably from 20 to 60 parts by mass, when the total mass of the ceramic raw materials is 100 parts by mass.

When the forming raw material is extruded, first, the forming raw material is kneaded to form a green body. The green body is then extruded to obtain a honeycomb formed product. The honeycomb formed body has a porous partition wall 15 that defines the cells 14 each extending from the fluid inflow end face 13 a to the fluid outflow end face 13 b, and is provided with the second flow path 16 at the center. The partition wall 15 of the honeycomb formed body is non-dried and non-fired partition wall 15.

(Production Step of Honeycomb Dried Body)

In the step of a honeycomb dried body, first, the resulting honeycomb formed body is dried to prepare a honeycomb dried body. The drying conditions are not particularly limited, and known conditions can be used. For example, it is preferable to dry the honeycomb formed body at a temperature of from 80 to 120° C. for 0.5 to 5 hours.

(Production Step of Honeycomb Body with Unfired Electrodes)

In the production step of a honeycomb body with unfired electrodes, first, an electrode forming slurry containing the ceramic raw material and water is applied to the side surface of the dried honeycomb body. The electrode forming slurry is then dried to form unfired electrodes to produce a honeycomb body with unfired electrodes.

For the honeycomb body with unfired electrodes, the dried honeycomb body is preferably provided with wide rectangular unfired electrodes each extending in a band shape in the extending direction of the cells 14, and also spreading in a circumferential direction. The circumferential direction refers to a direction along the side surface of the dried honeycomb body in the cross section orthogonal to the extending direction of the cells 14.

The electrode forming slurry used in the production step of the honeycomb body with unfired electrodes contains a ceramic raw material and water. The electrode forming slurry may contain a surfactant, a pore former, water, and the like.

As the ceramic raw material, it is preferable to use the ceramic raw material used when producing the honeycomb formed body. For example, when the main components of the ceramic raw material used when producing the honeycomb formed body are the silicon carbide particles and the metallic silicon, the silicon carbide particles and the metallic silicon may also be used as the ceramic raw materials of the electrode forming slurry.

A method of applying the electrode forming slurry to the side surface of the dried honeycomb body is not particularly limited. The electrode forming slurry may be applied, for example, by using a brush or by using a printing technique.

The electrode forming slurry preferably has a viscosity of 500 Pa·s or less, and more preferably from 10 to 200 Pa·s, at 20° C. The viscosity of the electrode forming slurry of 500 Pa·s or less can lead to easy application of the electrode forming slurry to the side surface of the dried honeycomb body.

After applying the electrode forming slurry to the dried honeycomb body, the electrode forming slurry can be dried to obtain unfired electrodes (the honeycomb body with unfired electrodes). The drying temperature is preferably from 80 to 120° C. The drying time is preferably from 0.1 to 5 hours.

(Production Step of Honeycomb Structure)

In the production step of a honeycomb structure, the honeycomb body with the unfired electrodes is fired to produce the honeycomb structure 1.

The firing conditions may be appropriately determined according to the types of the ceramic raw material used in the production of the honeycomb formed body and the ceramic raw material used in the electrode forming slurry.

Further, calcination is preferably carried out after drying the honeycomb molded body with the unfired electrodes and before the firing, in order to remove the binder and the like. The calcination is preferably carried out in an air atmosphere at a temperature of from 400 to 500° C. for 0.5 to 20 hours.

When the urea hydrolysis catalyst 40 is supported on the honeycomb structure 1, for example, the honeycomb structure 1 may be immersed in a container in which a slurry of the urea hydrolysis catalyst 40 is stored. By adjusting a viscosity of the slurry of the urea hydrolysis catalyst 40, a particle size of the urea hydrolysis catalyst 40 contained, and the like, the catalyst can be supported not only on the surfaces of the partition wall 15 but also in the pores of the partition wall 15, as well as an amount of catalyst to be supported can also be adjusted. Further, the amount of the catalyst to be supported can also be adjusted by sucking the slurry a plurality of times.

(2-2) Production of Reductant Injecting Device 100

The reductant injecting device 100 can be produced by inserting the honeycomb structure 1 into the outer cylinder 2, fixing the honeycomb structure 1 into the outer cylinder 2 via the insulation holding portion 23, and arranging the urea sprayer 3 at one end (inlet side end portion) of the outer cylinder 2, and then connecting each of the connectors 18 of the outer cylinder 2 to each of the electrode terminal protruding portions 17 arranged on the pair of electrode portions 12 via the electric wiring 19.

Next, a method of using the reductant injecting device 100 of the present embodiment will be described in detail.

(3) Method of Using Reductant Injecting Device 100

The reductant injecting device 100 according to the present embodiment sprays the aqueous urea solution used as a raw material from the urea sprayer 3 into the first flow path (cells 14) or the second flow path 16 depending on the temperatures of the exhaust gas. When the temperature of the exhaust gas is lower, the aqueous urea solution is selectively sprayed into the first flow path (cells 14) to decompose the urea in the aqueous urea solution in the first flow path (cells 14) to generate ammonia (reductant), and the ammonia is injected to the outside from the injection port 21. More specifically, the current is conducted through the honeycomb structure portion 11 to increase the temperature (heating), the aqueous urea solution is fed to the urea sprayer 3, and the aqueous urea solution is sprayed from the urea sprayer 3 to the first flow path (cells 14) of the honeycomb structure portion 11. In this case, by introducing the carrier gas from the carrier gas introduction port 22 to the fluid inflow end surface 13 a side of the honeycomb structure portion 11, the flow of the aqueous urea solution can be facilitated to prevent the aqueous urea solution from being stagnated in the first flow path (cells 14) of the honeycomb structure portion 11. The aqueous urea solution sprayed from the urea sprayer 3 enters the first flow path (cells 14) of the honeycomb structure portion 11 from the fluid inflow end face 13 a according to the flow of the carrier gas. The urea in the aqueous urea solution fed into the first flow path (cells 14) is decomposed by the temperature of the heated honeycomb structure portion 11 to generate the ammonia. On the other hand, when the temperature of the exhaust gas is higher, the aqueous urea solution is selectively sprayed into the second flow path 16 so that the aqueous urea solution is directly injected to the outside from the injection port 21. In this case, by introducing the carrier gas from the carrier gas introduction port 22 to the fluid inflow end surface 13 a side of the honeycomb structure portion 11, the flow of the aqueous urea solution can be facilitated to prevent the aqueous urea solution from being stagnated in the second flow path 16 of the honeycomb structure portion 11.

Here, the temperature of the exhaust gas is related to a required amount of the reductant. That is, when the temperature of the exhaust gas is higher, the required amount of the reductant is higher, and when the temperature of the exhaust gas is lower, the required amount of the reductant is lower. Therefore, for example, when the temperature of the exhaust gas is 200° C. or higher, the aqueous urea solution may be sprayed from the urea sprayer 3 into the second flow path 16 and injected to the outside as it is, and when the temperature of the exhaust gas is lower than 200° C., the aqueous urea solution may be sprayed from the urea sprayer 3 into the first flow path (cells 14), and the generated ammonia may be sprayed to the outside.

The amount of the aqueous urea solution fed is not particularly limited, and it may preferably be an amount such that an equivalent ratio of the ammonia to the amount of nitrogen oxides (NOx) contained in the exhaust gas is from 1.0 to 2.0. If the equivalent ratio is less than 1.0, the amount of nitrogen oxides discharged without purification may increase. However, if the SCR catalyst is provided with a NOx storage function, there may be a period during which the equivalent ratio is less than 1.0. If the equivalent ratio is more than 2.0, there is a risk that the exhaust gas is likely to be discharged with the ammonia mixed in the exhaust gas.

The aqueous urea solution is preferably an aqueous solution containing from 10 to 40% by mass of urea, although not particularly limited thereto. If the urea content is less than 10% by mass, it is necessary to spray a large amount of the aqueous urea solution in order to reduce NOx, and an amount of electric power required for conducting the current to heat the honeycomb structure portion 11 may increase. If the urea content is more than 40% by mass, there is a concern that the urea will solidify in cold regions. Preferable examples of the aqueous urea solution include AdBlue (an aqueous solution containing 32.5% by mass of urea; a registered trademark of Verband der Automobilindustrie (VDA)), which is widely distributed in the market.

The heating temperature of the honeycomb structure portion 11 is preferably 160° C. or higher, and more preferably from 160 to 600° C., and even more preferably from 250 to 400° C. The heating temperature of 160° C. or higher can lead to easy and efficient decomposition of the urea. The heating temperature of 600° C. or lower can allow the ammonia to be burned out and prevent the ammonia from being not fed to the exhaust gas pipe. Further, it is preferable that the heating temperature of the honeycomb structure portion 11 is 360° C. or higher because sulfur compounds such as ammonium hydrogen sulfate and ammonium sulfate precipitated on the reductant injecting device 100 can be removed.

The maximum voltage applied to the honeycomb structure portion 11 is preferably from 12 to 200 V, and more preferably from 12 to 100 V, and even more preferably from 12 to 48 V. The maximum voltage of 12 V or more can allow the temperature of the honeycomb structure portion 11 to be easily increased. The maximum voltage of 200 V or less can prevent a device for increasing the voltage from becoming expensive.

Embodiment 2

FIG. 3 is a schematic cross-sectional view showing a reductant injecting device according to Embodiment 2 of the present invention (a schematic cross-sectional view parallel to an extending direction of cells of a honeycomb structure portion). Further, FIG. 4 is a schematic cross-sectional view taken along the line B-B′ in FIG. 3 (a schematic cross-sectional view perpendicular to the extending direction of the cells of the honeycomb structure portion), and FIG. 5 is a schematic cross-sectional view take along the line C-C′ in FIG. 3 (a schematic cross-sectional view parallel to the extending direction of the cells of the honeycomb structure portion).

As shown in FIGS. 3 to 5 , in a reductant injecting device 200 according the present embodiment, the honeycomb structure 1 having the rectangular pillar shaped honeycomb structure portion 11 is housed in the outer cylinder 2, and in the outer cylinder 2, the second flow path (a space region 16) is provided at a position adjacent to the honeycomb structure 1. Further, the reductant injecting device 200 is provided with a control valve 31 as the spray direction switcher between the honeycomb structure 1 and the urea sprayer 3. Other configurations are the same as those of the reductant injecting device 100 of Embodiment 1. Therefore, descriptions of the other configurations will be omitted, and only differences will be described in detail.

In the reductant injecting device 200 according to the present embodiment, the first flow path through which the fluid can flow in the cells 14 of the honeycomb structure portion 11 and the second flow path placed adjacent to the first flow path, through which the fluid can flow, are provided inside the outer cylinder 2. Since the second flow path 16 is configured such that the fluid can flow through a portion other than the cells 14 of the honeycomb structure portion 11, the fluid flows through either the first flow path (cells 14) or the second flow path 16.

The control valve 31 used as the spray direction switcher is not particularly limited as long as it can change the spray direction of the aqueous urea solution. FIG. 5 shows a state when the control valve 31 controls the spray direction of the aqueous urea solution to the second flow path 16. Since the control valve 31 blocks the flow of the aqueous urea solution to the first flow path (cells 14) side, the aqueous urea solution can flow to the second flow path 16. On the other hand, if the flow of the aqueous urea solution to the second flow path 16 side is blocked by the control valve 31, the aqueous urea solution can flow to the first flow path (cells 14).

According to the reductant injecting device 200 of the present embodiment, which has the above structure, the aqueous urea solution can be sprayed from the urea sprayer 3 into the first flow path (cells 14) or the second flow path 16 depending on the temperatures of the exhaust gas, so that the same effects as those of the reductant injecting device 100 according to Embodiment 1 can be obtained.

Embodiment 3

FIGS. 6 and 7 are schematic cross-sectional views showing exhaust gas processing devices according to Embodiment 3 of the present invention.

As shown in FIGS. 6 and 7 , each of the exhaust gas processing devices 300 and 400 according to the present embodiment include: an exhaust gas pipe 41 through which the exhaust gas flows; the reductant injecting device 100 being configured to inject the ammonia or aqueous urea solution into the exhaust gas pipe 41; and an SCR catalyst 42 arranged at the exhaust gas pipe 41 on a downstream side of a position where the ammonia or aqueous urea solution is injected.

The exhaust gas pipe 41 is a pipe through which an exhaust gas (an exhaust gas containing NOx) discharged from various engines and the like is passed, and in which the exhaust gas and ammonia are mixed. The size of the exhaust gas pipe 41 is not particularly limited, and it can be appropriately determined depending on exhaust systems such as engines to which the exhaust gas processing devices 300 and 400 according the present embodiment are attached. The exhaust gas pipe 41 has a non-limiting length in the gas flow direction, but it preferably has a length in which a distance between the reductant injecting device 100 and the SCR catalyst 42 can be set to an appropriate distance.

A material of the exhaust gas pipe 41 is not particularly limited, but a material that is difficult to be corroded by the exhaust gas is preferable. Examples of the material of the exhaust gas pipe 41 include stainless steel.

In the reductant injecting device 100, the injection port 21 is attached to the exhaust gas pipe 41, which injects the ammonia or the aqueous urea solution into the exhaust gas pipe 41. Specifically, the reductant injecting device 100 sprays the aqueous urea solution into the exhaust gas pipe 41 when the temperature of the exhaust gas is 200° C. or higher, and it sprays the ammonia into the exhaust gas pipe 41 when the temperature of the exhaust gas is less than 200° C., whereby the ammonia or the aqueous urea solution is mixed into the exhaust gas in the exhaust gas pipe 41. In addition, the aqueous urea solution sprayed into the exhaust gas pipe 41 is decomposed into ammonia by the heat of the exhaust gas.

As shown in FIG. 6 , one reductant injecting device 100 may be attached to the exhaust gas pipe 41. Further, as shown in FIG. 7 , two reductant injecting devices may be attached to the exhaust gas pipe 41. In this case, the reductant injecting device on the downstream side may be the reductant injecting device 100 or a conventional reductant injecting device 500 (for example, the urea sprayer 3) that sprays the reductant (urea water) without heating. In a preferred embodiment, the reduction sprayer on the downstream side is the conventional reductant injecting device 500 that sprays the reductant (urea water) without heating. This is because when the amount of NOx in the exhaust gas is lower (when the temperature of the exhaust gas is lower), most of the NOx is purified by the reductant injecting device 100 and the SCR catalyst 42 on the upstream side, and when the amount of NOx in the exhaust gas is higher (when the temperature of the exhaust gas is higher), the urea water sprayed from the conventional reductant injecting device 500 is decomposed into ammonia by the temperature of the exhaust gas. Further, although not shown, three or more reductant injecting devices 100 may be attached to the exhaust gas pipe 41, as needed.

When the exhaust gas is used as the carrier gas to be introduced into the reductant injecting device 100, as shown in FIGS. 6 and 7 , the exhaust gas pipe 41 is branched upstream of the reductant injecting device 100, and a branched flow path 43 is connected to a carrier gas introduction port 22. On the other hand, when a carrier gas other than the exhaust gas (for example, an intake gas) is used, the carrier gas introduction port 22 is connected to a carrier gas feed source via a connection pipe or the like.

The SCR catalyst 42 in the form of a catalyst body (the honeycomb structure on which the SCR catalyst 42 is supported) is arranged at the exhaust gas pipe 41 on the downstream of the position where the ammonia or aqueous urea solution is injected. Therefore, as shown in FIG. 6 , when one reductant injecting device 100 is attached to the exhaust gas pipe 41, the SCR catalyst 42 is arranged at the exhaust gas pipe 41 on the downstream side of the position where the reductant injecting device 100 is attached. Further, as shown in FIG. 7 , when two reductant injecting devices are attached to the exhaust gas pipe 41, the SCR catalysts 24 are arranged at the exhaust gas pipe 41 on the downstream side of the positions where the reductant injecting device 100 and the conventional reductant injecting device 500 are attached, respectively.

Examples of the SCR catalyst 42 include vanadium-based catalysts and zeolite-based catalysts.

When the SCR catalyst 42 is used as a catalyst body supported on the honeycomb structure, it is preferable to contain the catalyst body in a container and attach the container to the exhaust gas pipe 41 on the downstream side.

The honeycomb structure that supports the SCR catalyst 42 is not particularly limited, and honeycomb structures known in the art can be used.

It is preferable that a filter for collecting particulate matters in the exhaust gas is arranged on the upstream side of the exhaust gas pipe 41. Examples of the filter for collecting particulate matters include a ceramic DPF (diesel particulate filter) 44 having a honeycomb shape. Further, it is preferable that an oxidation catalyst 45 for removing hydrocarbons and carbon monoxide in the exhaust gas is arranged on the upstream side of the exhaust gas pipe 41. The oxidation catalyst 45 is preferably in a state of being supported on a honeycomb structure made of ceramics (oxidation catalyst). Preferable examples of the oxidation catalyst 45 that can be used include precious metals such as platinum (Pt), palladium (Pd), and rhodium (Rh).

When one reductant injecting device 100 is attached to the exhaust gas pipe 41, the DPF 44 and the oxidation catalyst 45 are arranged at the exhaust gas pipe 41 on the upstream side of the position where the ammonia or the aqueous urea solution is injected by the reductant injecting device 100, as shown in FIG. 6 . Further, when two reductant injecting devices 100 are attached to the exhaust gas pipe 41, the DPF 44 and the oxidation catalyst 45 are arranged at the exhaust gas pipe 44 on the upstream side of the position where the ammonia or the aqueous urea solution is injected by the conventional reductant injecting device 500, and on the downstream side of the SCR catalyst 42 on the downstream side of the position where the reductant injecting device 100 is attached, as shown in FIG. 7 .

It is preferable to dispose an ammonia removing catalyst (oxidation catalyst) for removing ammonia on the downstream side of the SCR catalyst 42. Such a configuration can prevent ammonia from being discharged to the outside when excess ammonia that has not been used for removing NOx in the exhaust gas flows to the downstream side. Preferable examples of the oxidation catalyst arranged on the downstream side of the SCR catalyst 42 include precious metals such as platinum (Pt), palladium (Pd), and rhodium (Rh).

In the above description, the use of the reductant injecting device 100 of Embodiment 1 has been described. However, the reductant injecting device 200 of Embodiment may be used.

Embodiment 4

In a method for processing an exhaust gas according to Embodiment 4 of the present invention, when the temperature of the exhaust gas is 200° C. or higher, the aqueous urea solution is injected into the exhaust gas from the reductant injecting device 100, 200 of Embodiment 1 or 2, and the exhaust gas mixed with the aqueous urea solution is reduced with the SCR catalyst, and when the temperature of the exhaust gas is lower than 200° C., the generated ammonia is injected into the exhaust gas by the reductant injecting device 100, 200 of Embodiment 1 or 2, and the exhaust gas mixed with the ammonia is reduced with the SCR catalyst. The method for processing the exhaust gas can be easily carried out by using the exhaust gas processing device 300, 400 according to Embodiment 3.

When the temperature of the exhaust gas is higher (when the temperature of the exhaust gas is 200° C. or higher), the reductant injecting device 100, 200 selectively sprays the aqueous urea solution, and decompose the aqueous urea solution by the heat of the exhaust gas to generate ammonia, whereby the NOx can be purified by the SCR catalyst 42. Therefore, when the temperature of the exhaust gas is higher, it is not necessary to increase the generated amount of ammonia by the reductant injecting device 100, 200, so that the power consumption can be reduced and the generation of urea deposits can also be suppressed. On the other hand, when the temperature of the exhaust gas is lower (when the temperature of the exhaust gas is lower than 200° C.), the ammonia can be selectively sprayed to purify the NOx by the SCR catalyst 42.

Further, since the carrier gas can be fed together with the aqueous urea solution sprayed from the urea sprayer 3, the flow of the gas can be facilitated in the first flow path (cells 14) or the second flow path 16 of the honeycomb structure portion 11. As a result, it is possible to prevent urea from remaining in the first flow path or the second flow path, so that the generation limit of urea deposits can be increased. Further, when the temperature of the exhaust gas is lower than 200° C., the ammonia decomposed by heating can be discharged to the outside with the carrier gas, so that the reactivity can be improved.

When the ammonia is sprayed from the reductant injecting device 100, 200 (when the temperature of the exhaust gas is lower than 200° C.), a temperature and flow rate of the carrier gas and a power supplied to the honeycomb structure 1 are preferably adjusted such that a temperature of the fluid inflow end face 13 a of the honeycomb structure portion 11 is 150° C. or higher, and preferably 250° C. or higher. In order to perform such temperature control, for example, the temperature of the carrier gas is preferably 100° C. or higher, and the flow rate is preferably 10 L/min or more. The power supplied to the honeycomb structure 1 is preferably from 150 to 500 W.

An amount of the ammonia or the aqueous urea solution injected from the reductant injecting device 100, 200 is preferably such that the equivalent ratio of ammonia to the amount of nitrogen oxide contained in the exhaust gas is from 1.0 to 2.0. If the equivalent ratio is less than 1.0, the amount of nitrogen oxides discharged without purification may increase. If the equivalent ratio is more than 2.0, there is a risk that the exhaust gas is likely to be discharged with the ammonia mixed in the exhaust gas.

It is preferable that the sprayed amount of the aqueous urea solution and the temperature (power supply) of the honeycomb structure portion 11 are controlled by an electronic control unit. Further, the temperature may be calculated from a resistance value of the honeycomb structure portion 11, and the temperature of the honeycomb structure portion 11 may be controlled such that the calculated temperature is a desired temperature.

INDUSTRIAL APPLICABILITY

The reductant injecting device, the exhaust gas processing device, and the method for processing the exhaust gas according to the present invention can be suitably used for purifying NOx in the exhaust gases discharged from various engines and the like.

DESCRIPTION OF REFERENCE NUMERALS

-   1 honeycomb structure -   2 outer cylinder -   3 urea sprayer -   11 honeycomb structure portion -   12 electrode portion -   13 a fluid inflow end face -   13 b fluid outflow end face -   14 cell -   15 partition wall -   16 second flow path -   17 electrode terminal protruding portion -   18 connector -   19 electrical wiring -   21 injection port -   22 carrier gas introduction port -   23 insulation holding portion -   31 control valve -   41 exhaust gas pipe -   42 SCR catalyst -   43 branched flow path -   44 DPF -   45 oxidation catalyst -   100, 200 reductant injecting device -   300, 400 exhaust gas processing device -   500 conventional reductant injecting device 

The invention claimed is:
 1. A reductant injecting device, comprising: a honeycomb structure comprising: a pillar shaped honeycomb structure portion having a partition wall that defines a plurality of cells each extending from a fluid inflow end face to a fluid outflow end face; and at least one pair of electrode portions being configured to heat the honeycomb structure portion by conducting a current, the pair of the electrode portions being arranged on a side surface of the honeycomb structure portion, the honeycomb structure being configured to be able to decompose urea in an aqueous urea solution in the honeycomb structure portion heated by conducting the current to generate ammonia; an outer cylinder having an inlet side end portion and an outlet side end portion, the inlet side end portion comprising a carrier gas introduction port being configured to introduce a carrier gas, the outlet side end portion comprising an injection port being configured to inject ammonia, wherein the outer cylinder houses the honeycomb structure, and a first flow path through which a fluid can flow through the plurality of cells and a second flow path through which the fluid can flow from the inlet side end portion to the outlet side end portion without passing through the plurality of cells are provided in the outer cylinder; a urea sprayer being configured to spray the aqueous urea solution into the first flow path or the second flow path on an inflow side of the fluid, the urea sprayer being arranged at one end of the outer cylinder; and a spray direction switcher configured to be able to switch a spray direction of the aqueous urea solution to the first flow path or the second flow path depending on temperatures of an exhaust gas.
 2. The reductant injecting device according to claim 1, wherein the urea in the aqueous urea solution sprayed into the first flow path is decomposed in the first flow path by the honeycomb structure portion heated by conducting the current to generate ammonia, and the ammonia is injected to the outside, and wherein the urea aqueous solution sprayed into the second flow path is directly injected to the outside.
 3. The reductant injecting device according to claim 1, wherein the second flow path is configured such that the fluid can flow through a portion other than the plurality of cells of the honeycomb structure portion.
 4. The reductant injecting device according to claim 1, wherein the honeycomb structure portion is a hollow honeycomb structure portion configured such that the second flow path is located at a center in a cross section perpendicular to an extending direction of the cells.
 5. The reductant injecting device according to claim 1, wherein the second flow path is provided at a position adjacent to the honeycomb structure.
 6. The reductant injecting device according to claim 1, wherein the urea sprayer comprises a spray direction switcher, and wherein the spray direction switcher is a nozzle capable of changing a spray direction of the aqueous urea solution depending on feed pressures of the aqueous urea solution.
 7. The reductant injecting device according to claim 1, wherein the spray direction switcher is provided between the honeycomb structure and the urea sprayer, and the spray direction switcher is a control valve capable of changing the spray direction of the aqueous urea solution.
 8. The reductant injecting device according to claim 1, wherein the carrier gas is an exhaust gas.
 9. The reductant injecting device according to claim 1, wherein the honeycomb structure portion has an electrical resistivity of from 0.01 to 500 Ωcm.
 10. The reductant injecting device according to claim 1, wherein the honeycomb structure portion contains a silicon-silicon carbide composite material or silicon carbide as a main component.
 11. The reductant injecting device according to claim 1, wherein the honeycomb structure portion has a surface area per unit volume of from 5 cm²/cm³ or more.
 12. An exhaust gas processing device, comprising: an exhaust gas pipe through which an exhaust gas flows; the reductant injecting device according to claim 1, being configured to inject ammonia or an aqueous urea solution into the exhaust gas pipe; and an SCR catalyst arranged at the exhaust cylinder on a downstream side of a position where ammonia or the aqueous urea solution is injected.
 13. A method for processing an exhaust gas, the method comprising: injecting an aqueous urea solution into the exhaust gas from the reductant injecting device according to claim 1, and reducing the exhaust gas mixed with the aqueous urea solution by an SCR catalyst, when a temperature of the exhaust gas is 200° C. or higher; and injecting generated ammonia into the exhaust gas by the reductant injecting device, and reducing the exhaust gas mixed with the ammonia by the SCR catalyst, when a temperature of the exhaust gas is lower than 200° C. 